Whilst the following description discusses springs and spring clutches by illustration to their use with blinds, it should be understood that the invention is not so limited. Many designs of spring clutches have been manufactured in the past and in particular will be well known to persons skilled in the blind industry. Spring clutches are destined to provide a smooth translation of rotational movement of one shaft to another coaxial shaft. In the absence of such types of clutches, a direct translation is provided which can make the rotation difficult to initiate and control.
One known type of spring clutch has a first shaft, a helical spring and a second shaft. The helical spring has an internal diameter which is slightly smaller than the outer diameter of the first shaft. To install the spring, it is slightly expanded and slid onto the first shaft to grip the first shaft by frictional engagement. The second shaft is tubular and surrounds the first shaft with the helical spring interposed in between the shafts. The second shaft engages one end of the helical spring.
In use, the spring clutch may be mounted at one end of a blind roller so one of the shafts is fixed relative to the other. For example the first shaft is fixed and the second shaft is rotated by a pullcord engaging a pulley mounted upon one end of the first shaft. The second shaft is connected to the blind roller. Rotating the second shaft causes the helical spring to also rotate. Any resistance to rotation of the second shaft is first absorbed by rotational contraction of the helical spring and if the resistance exceeds a predetermined level the helical spring will slip about the first shaft. This ensures that excessive rotational torque in not applied to the roller blind and the blind will be smoothly rotated. Damage of the spring clutch can also be avoided.
Another type of spring clutch is disclosed in Australian Patent No. 557825. In this specification the problems of stairstepping and excessive torsional loading of spring clutches is discussed. A spring clutch is described which permits torque to be transmitted to the load (eg. a blind) in graduated steps to allow for a slower acceleration of the load than is the case when the maximum rated torque is applied suddenly to the load.
The spring clutch disclosed in that patent is characterised by having at least two helical springs frictionally engaging a first shaft. Each of the helical springs have tabs affixed to their ends which engage in openings in the surrounding coaxial second shaft. The openings have a configuration such that upon rotation of the first shaft, and consequently the helical springs, the tabs are sequentially engaged. This arrangement allows the first helical spring to contract to transmit torque to the second shaft prior to a second helical spring doing likewise. Further, the frictional engagement between the first helical spring and the first shaft is such that the second helical spring will not engage the second shaft unless the first helical spring is caused to slip about the shaft. This will occur if the torque applied to the first helical spring exceeds a predetermined limit. The second helical spring operates in a similar way as does a third helical spring and so on. Accordingly, a number of helical springs are sequentially used to transmit torque loads from the first shaft to the second shaft.
This patent also discloses a bidirectional spring clutch. To achieve this, both ends of each helical spring are provided with upstanding tabs. All the helical springs are provided with a common winding direction. Different ends are engaged depending upon the direction of rotation of the first shaft. A complex configuration of openings and barriers is necessary in the second shaft to achieve this functionality. Consequently such a bidirectional spring clutch is expensive to manufacture and assemble.